1. Field of the Invention
The present invention relates to spark plugs for use in internal combustion engines of motor vehicles, etc.
2. Description of the Related Art
Various types of sparks plugs are widely used to ignite a fuel mixture gas in a combustion chamber of an internal combustion engine mounted to a motor vehicle. For example, a spark plug is comprised of a center electrode and an earth electrode. A spark discharge gap is formed between the center electrode and the earth electrode. When a spark discharge is generated between the center electrode and the earth electrode of a spark plug mounted to a combustion chamber, a mixture gas of air and fuel is ignited. There is a spark plug having an improved structure in which an electrode chip is formed on at least one of the center electrode and the earth electrode in order to increase an ignition capability.
Recently, a temperature of a combustion chamber in an internal combustion engine in increased in order to improve the function of the internal combustion engine. Increasing the temperature of the combustion chamber requires superior wear resistance of the electrode chip formed on at least one of the center electrode and the earth electrode in the spark plug. There are spark abrasion or spark discharging wear and oxidation abrasion or oxidation wear which abrade the electrode chip in the spark plug. A surface of the electrode chip is instantaneously melted by the spark discharge when a spark abrasion occurs. A surface of an electrode in a spark plug is oxidized and vaporized in a high temperature environment when an oxidation abrasion occurs.
For example, iridium (Ir) is used as electrode material when an electrode chip is formed on at least one of a center electrode and an earth electrode of a spark plug because iridium has a high melting point and a superior spark discharging wear resistance. However, because iridium is a noble metal available on the commercial market at a high cost using iridium increases a manufacturing cost of the spark plug. In order to reduce the manufacturing cost, tungsten (W) is used instead of iridium because of having a higher melting point, when compared with iridium, and a superior spark wear resistance, and is available on the commercial market at low cost. However, because tungsten has a large chemical affinity with oxygen, tungsten has inadequate oxidation resistance. In order to avoid this problem, a patent document, a Japanese patent laid open publication No. H02-100281 has disclosed to use electrode material containing chromium (Cr) having a superior oxidation resistance in addition to tungsten.
However, the spark plug disclosed in Japanese patent laid open publication No. H02-100281 has the following problem.
In order to have an adequate oxidation resistance of the electrode chip, the electrode material contains chromium having a melting point of approximately 1857° C. which is lower than a melting point of approximately 3407° C. (or 3380° C.) of tungsten. Because increasing a content of chromium in the electrode chip decreases the melting point of the electrode chip, the electrode chip cannot provide an adequate spark discharging wear resistance. On the other hand, decreasing a content of chromium in the electrode chip can suppress decreasing of a melting point and a spark discharging wear resistance of the electrode chip. However, there is a possibility of it being difficult for the electrode chip to adequately maintain a necessary oxidation resistance.
There is accordingly a strong demand to provide a spark plug having an adequate spark discharging wear resistance and oxidation resistance with low manufacturing cost.